Light emitting diodes (LEDs) are solid-state devices that produce light by electroluminescence; that is, the optical phenomenon whereby a material emits light in response to an electric current. One common type of white light LED includes a phosphor powder blend mixed with a resin and deposited as a layer onto the surface of an optical emitter junction. The phosphor blend absorbs blue or UV wavelength light and reemits down-converted broadband green to red wavelength light.
As shown in FIG. 1, it is known to form a white light LED as a volumetric light emitting device 10 where the phosphor blend is molded into a three-dimensional or volumetric light conversion element 12. The volumetric light emitting device 10 includes a first reflector 14 including an aperture 16 for a light source 18 or an emitter junction, a second reflector 20 opposite the first reflector 14 for reflecting light emitted by the light source 18, and a volumetric light conversion element 12 extending between at least a portion of the first reflector 14 and at least a portion of the second reflector 20. The volumetric light conversion element 12 includes phosphor particles 22 dispersed in a resin to convert light emitted by the light source 18 from a first wavelength 24 to a second wavelength 26, the second wavelength 26 being longer than the first wavelength 24. In this way, the volumetric light emitting device 10 manages and distributes blue light and down-converted white light. Specifically, the volumetric light emitting device 10 radiates the down-converted light in a toroidal or spherical pattern. An example of a volumetric light-emitting device is illustrated by Brunt et al. U.S. Pat. No. 8,646,949.
In most LED lighting applications, the management of heat is critical to the operability of the lighting device. An LED maintaining a lower junction temperature has a longer life expectancy than an LED operated at a higher junction temperature. For lighting applications where LEDs are coated with phosphors, the heat generated by the LED and the heat generated by phosphor down-conversion is typically absorbed and transferred within a heat sink attached either directly to the LED or the printed circuit board (PCB) upon which the LED is mounted. In this way, conventional LED lighting applications rely on the design of the heat sink to maintain the life expectancy of the light source by maintaining a desired LED's junction temperature.
In a volumetric light emitting device 10 of the type described in FIG. 1, the phosphor material that forms the volumetric light conversion element 12 is not located on or directly adjacent to the LED light source 18 and the corresponding heat sink (not shown, but either directly attached to the LED light source 18 or the PCB 19). Consequently, much of the heat generated from down-conversion does not transfer efficiently to the heat sink.
Referring now to FIG. 2, a hot spot 28 can be formed in the volumetric light emitting device 10 as shown. Light with a first wavelength 24 emitted from the LED light source 18 passes through a clear encapsulant 17, excites a phosphor 22, 30 that emits light of a down-converted second wavelength 26 within the volumetric light conversion element 12 and generates heat. The heat will then generally find an exit path through the resin material in which the phosphors 22, 30 are embedded, transfer to the sides and top of the volumetric light conversion element 12 and dissipate through convection, conduction or radiation.
The hot spot 28 is a localized volume in the volumetric light conversion element 12 where a concentration of down-conversion creates a localized higher temperature with respect to the overall average temperature of the volumetric light conversion element 12. A higher localized temperature may prematurely degrade the phosphor materials, the resin material, and encapsulant. Furthermore, phosphors become less efficient at higher temperatures emitting less light than at lower temperatures. The hot spot 28 occurs, in part, because the phosphors 26, 30 are dispersed in a homogenous fashion throughout the resin that defines the volumetric light conversion element 12. The host resin material may include many materials, such as a hardened silicone. The thermal conductivity for such a host material is typically much less than the thermal conductivity of the heat sink material, which is often aluminum or a highly thermally conductive thermoplastic. Hot spots are especially problematic if the heat created exceeds the material property specification of the resin material, the encapsulant or the phosphor materials.
Despite the homogeneous distribution of the phosphors in the volumetric light conversion element 12, the distribution of the excitation of the phosphors 22, 30 in the volumetric light emitting device 10 is not homogeneous throughout the volumetric light conversion element 12. Rather, there is a higher concentration of phosphor excitement where the light of the first wavelength 24 from the LED light source 18 initially contacts the phosphors 30 at the interface 34 between encapsulant 17 and the volumetric light conversion element 12. With an axially symmetric volumetric light emitting device 10, where the axis of symmetry 32 is vertical and intersects the center of the LED light source 18, the area of the interface 34 is defined by a circle. The volume defined by the area of the interface 34 and extending along the axis of symmetry 32 forms the previously described hot spot 28.
For example, a typical volumetric light emitting device 10 may have an interface 34 with a radius of approximately 4.45 millimeters (mm) and a corresponding area of approximately 62.2 mm2. As much as 30 to 70 percent of the down-conversion from a first wavelength to a second wavelength may occur in the volume described approximately by the area of the interface 34 extending approximately 3 mm vertically along the axis of symmetry 32.
Additionally, the high concentration of phosphor excitement at the interface 34 just above the encapsulant 17 results in down-converted light 36 traveling back down into the area of the LED light source 18. The down-converted light 36 heats the LED light source 18, shortening the life expectancy of the LED light source 18. By not exiting through the exterior walls of the volumetric light conversion element 12, the down-converted light 36 directed back at the LED light source 18 contributes to a reduction in the overall efficiency of the volumetric light emitting device 10.
The maximum temperature inside the volumetric light conversion element 12 depends on several factors including the output power of the LED light source 18, the interface area between the encapsulant and the phosphors, the type of phosphors 22, 30, the thermal conductivity of the materials, the exterior geometry, and the ambient temperature surrounding the volumetric light conversion element 12. Many of the LED light sources used today have very high power densities emitting a lot of light into a very small area. To prolong the life expectancy and maximize efficiency of the volumetric light conversion element 12, the volumetric light emitting device 10 may include elements to mitigate the overall heat generated by the down-conversion from a first wavelength to a second wavelength and minimize areas where hot spots are created. Increasing the area of the interface between the encapsulant material and the phosphor particles dispersed in resin increases the volume where the light of the first wavelength from the LED light source initially contacts the phosphor. As the light excites the phosphors, the heat generated by down-conversion may disperse over a wider area, thus minimizing the intensity and volume of a potential hot spot. Additionally, the increased interface area reduces the amount of down-converted light directed back into the LED light source.